Fix caching when moving script
When caching the result of a `map` function, the hash that is computed depends on many properties of this function, such as all the python objects it uses, its code and also the location of this code.

Using the full path of the python script for the location of the code makes the hash change if a script like `run_mlm.py` is moved.

I changed this by simply using the base name of the script instead of the full path.

Note that this change also affects the hash of the code used from imported modules, but I think it's fine. Indeed it hashes the code of the imported modules anyway, so the location of the python files of the imported modules doesn't matter when computing the hash.

Close https://github.com/huggingface/datasets/issues/2825